Pressure-wind relationship calculations for tropical cyclones#Knaff-Zehr-Courtney

A tropical cyclone's maximum sustained wind and minimum central air pressure are interlinked and can be used to describe a tropical cyclone's intensity.{{cite web |last1=Rosendal |first1=Hans D. |last2=Shaw |first2=Samuel L. |title=Relationship of Maximum Sustained Winds to Minimum Sea Level Pressure in Central North Pacific Tropical Cyclones |url=https://repository.library.noaa.gov/view/noaa/7039/noaa_7039_DS1.pdf |publisher=National Weather Service |format=PDF |date=February 1982}} While the maximum winds are more closely related to the destructive potential of a tropical cyclone, it is harder to reliably measure. According to Christopher Burt from Weather Underground, the most reliable method of estimating pressure from wind involves using the Dvorak Technique with an infrared image, which shows how cold cloud tops are.{{Cite web |last=Burt |first=Christopher C. |date=2017-11-02 |title=Possible New World Record for Lowest Barometric Pressure: 860 mb? |url=https://www.wunderground.com/cat6/possible-new-world-record-lowest-barometric-pressure-860-mb |access-date=2024-12-22 |website=Weather Underground |language=en}} Joe Courtney and John Knaff noted that as several models are based on Atlantic data, it can lead to biases in other parts of the world.{{Cite journal |last1=Courtney |first1=Joe |last2=Knaff |first2=John A. |date=2009-11-03 |title=Adapting the Knaff and Zehr wind-pressure relationship for operational use in Tropical Cyclone Warning Centres |url=https://www.researchgate.net/publication/236076649 |journal=Australian Meteorological and Oceanographical Journal |volume=58 |issue=03 |pages=167–179 |doi=10.22499/2.5803.002 |via=ResearchGate}}

Most pressure-wind models are in the form of:{{Cite journal |last=Holland |first=Greg |date=2008-09-01 |title=A Revised Hurricane Pressure–Wind Model |url=https://journals.ametsoc.org/view/journals/mwre/136/9/2008mwr2395.1.xml?tab_body=pdf |journal=Monthly Weather Review |language=EN |volume=136 |issue=9 |pages=3432–3445 |doi=10.1175/2008MWR2395.1 |issn=1520-0493}}

$v\_m=a\backslash Delta\; p^x$

Where v_m is the maximum wind, Δp is the change in pressure from an external point to the center. a and x are constants. Ted Fujita was the first to modify the exponent; before then, it mostly stood at 0.5.

{{Needs expansion|date=January 2025}}

Knaff and Zehr (2007) came up with the following formula to relate wind and pressure, taking into account movement, size, and latitude:{{Cite web |last=Holland |first=Greg |year=2017 |title=Global Guide to Tropical Cyclone Forecasting: Chapter Eleven |url=https://cyclone.wmo.int/pdf/Chapter-Eleven.pdf |access-date=2024-12-28 |website=World Meteorological Organization}}

$MSLP=23.286-0.483\; V\_\{srm\}-(\; \backslash frac\{V\_\{srm\}\}\{24.254\}\; )-12.587S-0.483\backslash phi+P\_\{env\text{'}\}$

Where V_srm is the max wind speed corrected for storm speed, phi is the latitude, and S is the size parameter. S is more specifically defined as the ratio of tangential wind at a radius of {{Convert|500|km|mi}} to its value under a Rankine vortex model.

In 2008, Greg Holland published his model to the Monthly Weather Review.

Joe Courtney and John A. Knaff published in 2009 a correction to the previous Knaff-Zehr model. They noted that the Knaff-Zehr model had issues with calculating for storms at low latitudes. The equation derived is:

$P\_c=23.286\; -\; 0.483V\_\{srm1\}\; -\; (V\_\{srm1\}/24.254)^2\; -\; 12.587S\; -\; 0.483\backslash Phi\; +\; P\_e$

The interchangeability of pressure and wind allows for the two to be used to give equivalencies for the public. Pressure-wind relations can be used when information is incomplete, forcing forecasters to rely on the Dvorak Technique.{{Cite journal |last1=Kieu |first1=Chanh Q. |last2=Chen |first2=Hua |last3=Zhang |first3=Da-Lin |year=2010 |title=An Examination of the Pressure–Wind Relationship for Intense Tropical Cyclones |url=https://www2.atmos.umd.edu/~dalin/Kieu-Chen-Zhang-PW-TCs-WAF10.pdf |journal=Weather and Forecasting |volume=25 |issue=3 |pages=895–907 |doi=10.1175/2010WAF2222344.1 |via=University of Maryland Department of Atmospheric and Oceanic Science}}

Some storms may have particularly high or low pressures that do not match with their wind speed. For example, Hurricane Sandy had a lower pressure than expected with its associated wind speed.{{Cite journal |last1=Chavas |first1=Daniel R. |last2=Reed |first2=Kevin A. |last3=Knaff |first3=John A. |date=2017-11-08 |title=Physical understanding of the tropical cyclone wind-pressure relationship |journal=Nature Communications |language=en |volume=8 |issue=1 |pages=1360 |doi=10.1038/s41467-017-01546-9 |pmid=29118342 |issn=2041-1723|pmc=5678138 }}

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Category:Tropical cyclone meteorology

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Category:Physical modeling